Rotational drive device

Abstract

The structure for detecting tooth-skipping of the speed reducer of the rotary driver is reduced in weight and size. In the rotary driver the occurrence of tooth-skipping is detected based on the difference in outputs from the encoders located at the input side (the side of the motor) and at the output side (the side of the load), which is opposite the input side in relation to the speed reducer. The rotary driver comprises a motor, a speed reducer located between the motor and a load to reduce the rotary speed of a rotary shaft at the side of the motor, to thereby transmit the reduced rotary speed to a rotary shaft at the side of the load, a first encoder for detecting a rotation of the rotary shaft at the side of the motor, a second encoder for detecting a rotation of the rotary shaft at the side of the load, a section for detecting any difference between a first detected value that is obtained by dividing an output of the first encoder by a rate for reducing the speed by the speed reducer and a second detected value that is obtained from an output of the second encoder, and a section for detecting tooth-skipping that detects tooth-skipping of the speed reducer based on the difference.

The invention claimed is: 
     
       1. A rotary driver comprising:
 a speed reducer located between a motor and a load, the reducer reducing a rotary speed of a rotary shaft at a side of the motor, the reducer transmitting the reduced rotary speed to a rotary shaft at a side of the load; 
 a first encoder for detecting a rotation of the rotary shaft at the side of the motor; 
 a second encoder for detecting a rotation of the rotary shaft at the side of the load; 
 a section for detecting difference that obtains a difference between a first detected value that is obtained by dividing an output of the first encoder by a rate for reducing the speed by the speed reducer and a second detected value that is obtained from an output of the second encoder; and 
 a section for detecting tooth-skipping that detects tooth-skipping of the speed reducer based on the difference; 
 wherein the section for detecting the tooth-skipping compares the difference with a predetermined threshold; and 
 wherein the section for detecting the tooth-skipping detects tooth-skipping of the speed reducer if the difference is greater than the threshold or if the difference is equal to, or greater than, the threshold. 
 
     
     
       2. The rotary driver of  claim 1 , wherein the section for detecting the tooth-skipping determines the difference by subtracting the second detected value from the first detected value;
 wherein the threshold includes a positive first threshold that is compared with the difference if the first detected value is greater than the second detected value or if the first detected value is equal to, or greater than, the second detected value and a negative second threshold that is compared with the difference if the first detected value is smaller than the second detected value or if the first detected value is equal to, or smaller than, the second detected value; 
 wherein plus tooth-skipping wherein a number of rotations at the side of the motor is greater than a number of rotations at the side of the load is detected if the difference is greater than the first threshold or if the difference is equal to, or greater than, the first threshold in a positive side; and 
 wherein minus tooth-skipping wherein the number of rotations at the side of the motor is smaller than the number of rotations at the side of the load is detected if the difference is smaller than the second threshold or if the difference is equal to, or smaller than, the second threshold in a negative side. 
 
     
     
       3. The rotary driver of  claim 1 , further comprising a section for correcting the difference to correct the difference;
 wherein the section for correcting the difference obtains an amount for correcting the difference that corresponds to a number of skipped teeth from the difference so as to obtain a corrected difference by correcting the difference by using the amount for correcting the difference. 
 
     
     
       4. The rotary driver of  claim 3 , wherein the difference is a value that accumulates absolute values of the difference in a predetermined period. 
     
     
       5. The rotary driver of  claim 3 , further comprising:
 a section for estimating torque that estimates torque by multiplying the corrected difference in a rotated angle by torsional stiffness K of the speed reducer, the corrected difference in a rotated angle being obtained by converting the corrected difference to an angle; 
 a section for controlling the angle that generates a designated torque so as to bring a rotated angle of the rotary shaft at the side of the load close to a designated angle based on a rotated angle of the rotary shaft at the side of the load, which rotated angle is obtained by converting the second detected value to an angle, or based on the rotated angle and the corrected difference in a rotated angle; 
 a section for controlling the torque that generates a designated current for bringing the estimated torque close to the designated torque based on difference between the designated torque and the estimated torque; and 
 a section for controlling current that supplies current to the motor based on the designated current. 
 
     
     
       6. The rotary driver of  claim 3 , further comprising:
 a section for estimating torque that estimates torque by multiplying the corrected difference in a rotated angle by torsional stiffness K of the speed reducer, the corrected difference in a rotated angle being obtained by converting the corrected difference to an angle; 
 a section for controlling the torque that generates a designated current for bringing the estimated torque close to the designated torque based on difference between the designated torque and the estimated torque; and 
 a section for controlling current that supplies current to the motor based on the designated current. 
 
     
     
       7. The rotary driver of  claim 1 , further comprising a section for correcting the detected value that corrects the detected value;
 wherein the section for correcting the detected value obtains an amount for correcting the detected value from the difference, the amount for correcting the detected value corresponding to a number of tooth-skipping or a value that accumulates the number of tooth-skipping; 
 wherein, if the first detected value is a counted number or an angle detected by the first encoder, the counted number or the angle detected by the first encoder is corrected by the amount for correcting the detected value; 
 wherein, if the first detected value is a rotated angle that is obtained by converting a number counted by the first encoder to a rotated angle, a rotated angle of the rotary shaft at the side of the motor is corrected by the amount for correcting the detected value; and 
 wherein the section for detecting the difference obtains the difference of the counted number or of the detected angle based on the corrected counted number or the corrected detected angle or obtains the difference of the rotated angle based on the corrected rotated angle. 
 
     
     
       8. The rotary driver of  claim 7 , further comprising:
 a section for estimating torque that estimates torque by multiplying a difference in a rotated angle or a corrected difference in a rotated angle by torsional stiffness K of the speed reducer, the difference in a rotated angle being obtained by converting the corrected counted number to an angle, the corrected difference in a rotated angle being obtained by converting the corrected difference to an angle; 
 a section for controlling an angle that generates a designated torque for bringing a rotated angle of the rotary shaft at the side of the load close to a designated angle based on a rotated angle of the rotary shaft at the side of the load, which rotated angle is obtained by converting the corrected counted number to an angle, or the rotated angle of the rotary shaft at the side of the load and the designated angle; 
 a section for controlling the torque that generates a designated current for bringing the estimated torque close to the designated torque based on a difference between the designated torque and the estimated torque; and 
 a section for controlling current that supplies current to the motor based on the designated current. 
 
     
     
       9. The rotary driver of  claim 1 , further comprising a section for predicting a lifetime that predicts a lifetime of the speed reducer based on a result detected by the section for detecting the tooth-skipping;
 wherein the section for predicting the lifetime comprises:
 a memory section that stores a predetermined accumulated number of tooth-skipping that is determined based on an operational record; and 
 a section for prediction that predicts a lifetime of the speed reducer from an accumulated number of tooth-skipping that is detected by the section for detecting the tooth-skipping based on the predetermined accumulated number that is stored in the memory section.